Medium-chain conveyors, and double medium-chain conveyors in particular, have proved to be a versatile and efficient transport means over decades in black coal mining, where they have been generally used in underground longwall working.
Underground longwall working was introduced from Europe into North America about 20 years ago. Today, more than half the underground black coal production is achieved using longwall working ("longwalling"). North American longwalling has in the meantime far outrun European longwalling with respect to output, face output, face lengths and lengths of working panels (also as a result of the less complex geological features). The coal winning, transport and excavation systems have been adapted to the increased demands of the North American market, whereas the individual components of the transport systems, such as conveyor chains, scrapers and chain wheels of chain scraper conveyors, for example, have not.
Modern American shearer loaders achieve production levels of more than 2000 t/h with cutting depths of 1 m (40 in) and more, and at speeds of travel of about 15 m/min (260 ft/min) as well as with usual seam thicknesses of about 2 m (78 in). These mass flows must be transported using chain scraper conveyors. Although the size of the available conveyors with troughs of standardised width and height, and the speeds of the chain belt systems are adequate to cope with these mass flows, problems arise with the chain belt system and the corresponding chain drive wheels in respect of the following:
the chain thicknesses necessary with the given drive power as a result of the high mass flow, presently up to 1350 kW (1800 hp), under the confined geometric conditions of the conveyor; PA1 the extreme wear on the scraper tips as a result of the almost uninterrupted movement of the scrapers under load through the trough sections of the scraper conveyors, and PA1 the extreme wear on the side links of the chain wheels as a result of non-positive and positive power transmission at a very high level and as a result of wear on the chain pitch occurring after only a short time in operation. The pitch of the chain wheel side links no longer matches the pitch of the chain link to be driven, and therefore the chain connectors and scrapers on the chain strand protrude from the horizontal links of the chain belt, which they are fastened to, between the teeth against the chain wheel axis and, as wear of the chain wheel side links on the tooth flanks lying in the direction of rotation increases, come into contact with the root of the teeth. This causes wear on the root or base of the chain wheel teeth which leads to these weakening and often prematurely snapping off unchecked. PA1 The hardness of the curved area of the horizontal links and the vertical links is greater than that of the shank. Therefore, the curved area has a higher resistance to pitch wear and to wear at the external curved area of the horizontal links resulting from interaction between chain and chain wheel. The softer shanks acquire their working capacity and the required elasticity from the chain link. This technique known per se is also used in this new chain belt system because of its advantages of application. PA1 The wear characteristics of the chain wheel side links are substantially improved by an optimised contact surface between the chain and chain wheel side link. It is particularly advantageous here that the flat chain used as horizontal link has a standard chain link, merely having shortened pitch, and also has a considerably larger external width than the vertically mounted flat links. In this, the present chain belt system also differs from other systems, in which both chain links are flat. The horizontal link is the chain link driven by the chain wheel side link. The length of contact area between the nose or curve of the horizontal link and the correspondingly curved contact surface on the chain side link is thus increased in comparison to chains with flat horizontal links by about 10 mm. Since a chain, which is thicker by a nominal size, comes into operation with the same pitch as the next-lower chain thickness, the height of the contact area of the thicker horizontal link in the chain side link is also increased by 4 mm. A further increase in contact surface and a resulting reduction in the contact pressure may be achieved by providing as narrow a slot as possible between the chain wheel side links to accommodate the vertically mounted flat links. If reshaping of the round steel chain links by forging to form flat links results in correspondingly narrow tolerances, then the manufacturing tolerances for the width of the flattened shanks of the vertical links may also be kept extremely low. If production is correspondingly accurate, therefore, the dimensions of the slot for the vertical links on the chain side links may be reduced by about 4 mm. This results in an overall improvement of the contact surface for the horizontal links on the chain wheel side link of about 40%. PA1 Considering that chain wheels are almost exclusively operated in one direction of rotation, symmetrical assembly of the chain wheel teeth is completely unnecessary. An asymmetrical tooth configuration in the direction of travel is more likely to enable a considerably larger free space to be created between the root of the tooth and the scraper or chain connector. If material is removed from the tooth flank below the chain wheel side link in the direction of travel in front of the horizontal link and material is added to the opposite side, i.e. if the tooth shape is made asymmetrical by adding wear material to the contact surface with the horizontal link and a corresponding interstice on the opposite tooth flank, the free space between the root or base of the tooth and the chain connectors or scrapers, which are fastened to the horizontal links of the chain belt system, may be increased to about 25 to 30 mm, without weakening the tooth. Wear may be made apparent by one or several lateral marks next to the chain wheel side link on each tooth. If wear greater than the assumed magnitude is discovered in the new chain belt, then there still remains sufficient time, even in the case of an exponential rate of wear, to arrange the necessary change for the next maintenance or weekend shift. PA1 The practically uninterrupted movement of the scrapers through the trough section of the conveyor under load of coal and quartz mining material causes rapid wear of the scraper tips both along the length and height of the scrapers. The consequence of this is that the overlap of the trough section over the rim or end of the scraper tips is no longer adequate and the chain belt is disengaged from the trough section guide. For this reason, it was necessary, inter alia, to completely replace the scrapers of a chain belt long before the chain had reached the end of its expected service life.
An additional factor that complicates matters in particular here is that the wear on the chain wheel side links is not linearly dependent on their time in operation or on the quantity of coal transported, but increases exponentially as a result of the side links hardening on use after the hard protective layer has pierced the soft base material. This also increases the risk of wear on the root of the chain wheel teeth, which means that chain wheels frequently have to be changed during production shifts, and not during maintenance and weekend shifts, as scheduled. This leads to costly and unscheduled production outages.
One possibility of at least partly eliminating these difficulties is to increase the chain thicknesses, taking into account the given geometric restrictions of the available conveyors.
The available conveyors present two different geometric restrictions with respect to chain thicknesses:.
1. The height of the conveyor trough section restricts the outer width of the vertically mounted chain link, in particular in relation to the lower run (the return channel of the chain belt). PA0 2. The pitch circle diameter of the chain wheel is a function of the pitch and the link diameter of the chain used, whereby the chain pitch has a substantially greater influence. The pitch circle diameter is, however, decisive in respect of the size of the drive frame. In the confined conditions underground, the pitch circle diameter has an even greater influence on the angle of the chain belt on its entry and exit from the drive frame. The greater the pitch circle diameter with a given drive frame length, the greater the angle of the chain belt becomes, and this leads to a further increase in wear on the profiles of the drive frame and the scrapers as well as to additional frictional resistances. PA0 1. the vertically mounted chain link of the chain belt system is flattened on its outer width by forging, while retaining its original cross-sectional area, and PA0 2. the pitch of both the welded horizontal links and of the forged vertical flat links is reduced to the next-lower nominal size, for example, in accordance with DIN 22252. PA0 1. Chain pitch wear is taken into account by the following measures: PA0 2. Chain wheel wear is handled in the following way: PA0 3. Therefore, the following must be taken into consideration with respect to wear on the scraper tips:
The following applies as regards chain wheel wear:
Chain wheel wear can only be conditionally counteracted, as the complicated chain wheel geometry and the casting materials used do not allow the application of heat treatment processes similar to those used in the production of chains and scrapers. The hardness penetration depths usual at present for chain wheel side links of 6 to 8 mm can not therefore be increased any further, since the base material of the chain wheel teeth must have a minimum elasticity to protect them against snapping. An additional difficulty in the case of chain belt systems with reduced pitch is that the width of the teeth in peripheral direction of the chain wheel is further reduced by the short pitch of the chain.
However, the chain wheel must not be left out of the consideration regarding centre-strand conveyor systems under high load. The ratio of the number of contacts between an individual chain link and a chain wheel side link to the number of contacts between an individual chain wheel side link and the chain links to be driven, which is about 1:350 with usual face lengths, is in itself sufficient basis for optimisation considerations.
A further important consideration with a view to improving the corresponding chain wheel is to permit the time the chain wheel is replaced to be changed and planned. As already mentioned above, the assumed wear for chain wheels is about 6 to 8 mm on the basis of the hardness penetration depth of the chain wheel side links or teeth. Monitoring of this low assumed value is only possible at considerable expense in underground mining operations. For exact wear measurements, the chain would have to be removed from the chain wheel in a time-consuming exercise for the side links to be checked. Things are different in practice: wear measurements are not carried out. Consequently, the time for replacement is not recognised in good time. The necessity to replace the chain wheel is more likely to be realised when about 10 to 15 mm of wear is clearly visible on its outside. By this time, however, the scrapers and the chain connectors are already touching the base of the chain wheel teeth. This not only causes mechanical damage to the chain wheel teeth resulting in some of the teeth snapping off, but also leads to considerable mechanical damage to the chain connectors and scrapers, which subsequently result in unchecked shutdowns and faults.